Sidedress Nitrogen After Manure and Rain on a Sandy Loam
A 90-acre field of corn in central Iowa is at V5 on June 5. The field is a sandy loam with 2.1% organic matter and a history of early-season nitrate loss. It was fall-applied 4,000 gal/ac of swine lagoon effluent analyzed at 15 lb plant-available N per 1,000 gal. Starter fertilizer supplied 20 lb N/ac. The previous crop was corn. Yield goal is 180 bu/ac. A pre-sidedress nitrate test (PSNT, 0-12 in.) from 15 cores averages 14 ppm nitrate-N. The 5-day forecast calls for 2.5 inches of rain, with more the following week. The grower has 28% UAN and a sidedress applicator with coulter injection, but no urease inhibitor on hand. He asks whether to sidedress, at what rate, and whether to split.
1. Build an approximate N budget, interpret the PSNT, and recommend a sidedress rate and timing.
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N requirement: 180 bu/ac x 1.1 lb N/bu = about 198 lb N/ac. Credits: starter 20 lb + manure 4,000 gal x 15 lb/1,000 gal = 60 lb + PSNT 14 ppm x 4 lb/ac per ppm (0-12 in.) = 56 lb. Total credits = 136 lb. Residual need = about 62 lb N/ac. The PSNT is below the 20-25 ppm critical range, so the field is responsive. But the credits are not all immediately available: the manure N is only partly in the nitrate pool, and 2.5 in. of rain on a sandy loam can move nitrate below the root zone. Do not apply the full 62 lb before the rain. Inject 35-40 lb N/ac now as 28% UAN with coulters, then tissue-test or use a chlorophyll meter at V8-V10 and apply the remaining 20-25 lb if the crop is pale. Coulter injection reduces volatilization loss; without a urease inhibitor, avoid surface broadcasting.
2. The rain event becomes 5 inches over 36 hours, and the field is saturated for two days. How should the recommendation change?
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Saturated soil creates denitrification risk as well as leaching, especially in the finer pockets of the sandy loam. Nitrate already in the soil and any UAN applied before the storm may be lost. Wait until the field is trafficable, then take a 0-12 in. PSNT plus a tissue sample. If the PSNT has dropped below 10 ppm and corn is at V6-V8, sidedress 50-60 lb N/ac rather than the original 35-40. Do not add more N while the soil is saturated; the crop cannot take it up and loss continues. Consider a nitrification inhibitor only if the label allows and the product can be applied with the sidedress rig. Keep the total seasonal N near the 198 lb budget; extra N beyond that raises leaching risk without likely yield benefit.
3. The grower proposes surface-broadcasting the entire 60 lb N/ac as UAN without incorporation to cover more acres before the rain. Evaluate that decision.
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Surface-broadcast UAN without incorporation is the weakest option here. UAN contains urea, and urea hydrolyzes on the surface; with warm temperatures and no incorporation, ammonia volatilization can cost 10-25% of the N, more on moist residue. The forecast rain could incorporate it, but 2.5 in. is also enough to move nitrate past the root zone on a sandy loam. Coulter injection places N in the root zone and avoids volatilization, so it is worth the slower pace. If the grower must broadcast, do it immediately before a modest rain of 0.25-0.5 in., not before a 2.5-in. event, and reduce the rate to 40 lb N/ac. Keep the remaining N for a V8 sidedress based on tissue N.